Thin film transistor having a short channel formed by using an exposure mask with slits

ABSTRACT

A mask containing apertures therein which is used for fabricating a channel of a thin film transistor (TFT), wherein the pixel charging time for a TFT in a high-resolution liquid crystal display (LCD) device is reduced by minimizing the length of the channel in the TFT when the active region is made of amorphous silicon. The length of the channel can be minimized by exposing light through the apertures in an exposure mask when forming the channel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating a thin filmtransistor (TFT) for a TFT liquid crystal display (hereinafter, referredto as a TFT-LCD) device and in particular, to a method for fabricating aTFT, which allows the amount of charge current supplied to the pixels tobe increased. This is achieved by fabricating a channel of a TFT formedat an amorphous silicon active region so that its length is not greaterthan 4 μm. The channel according to the present invention is formed byusing a mask having slits or other slit-type openings.

2. Description of the Background Art

In high resolution TFT-LCD devices, due to the TFT structure having gateelectrodes with narrow widths, the amount of charge current supplied tothe pixels via the TFT is relatively small and a relatively longercharging time is required compared to low resolution TFT-LCD devices. Ina typical TFT, where an amorphous silicon is used for an active region,there are limits to improving the mobility of the electric chargesbecause the mobility does not generally exceed 0.6 cm/Vs for amorphoussilicon. Thus, to improve the mobility of electric charges, the lengthof the channel (formed between the source and drain regions, and abovethe gate electrode) has to be shortened. However, because thefabrication process of a TFT-LCD is performed on a substrate having alarge surface area, it is not easy to apply conventional semiconductorfabrication techniques for TFT-LCD fabrication requiring a short lengthchannel. For example, it is difficult to form a uniform photoresist andto properly perform photolithography on a large substrate area to obtaina short length channel.

For defining a channel region according to the conventional art, aconventional light transmittance mask (i.e., exposure mask) is used byrelying upon the resolution of an exposure device. The method forfabricating a TFT according to the conventional art will be described indetail with reference to accompanying drawings.

FIGS. 1A-1D are sectional views sequentially illustrating the method forfabricating a TFT according to the conventional art.

As a first step (FIG. 1A), a gate electrode 2 is formed on a glasssubstrate 1, and a gate insulating layer 3, an amorphous silicon 4, andan n⁺ amorphous silicon 5 having a high density n-type ions injectedtherein are sequentially formed thereon. Then, patterning of the n⁺amorphous silicon 5 and the amorphous silicon 4 is achieved by aphotolithography process to form an active region above the gateinsulting layer 3 corresponding to the upper portion of the gateelectrode 2.

As a second step (FIG. 1B), a source 6A and a drain 6B are formed bydepositing a metal on the upper surface of the above structure in FIG.1A and then patterned to form respective portions separated by a certaindistance from the center portion of the n⁺ amorphous silicon 5. Thesource and drain 6A, 6B extend over the end portions of the amorphoussilicon 4 and the n⁺ amorphous silicon 5, and onto a portion of the gateinsulating layer 3. Here, portions of the n⁺ amorphous silicon 5 exposedbetween the source and drain portion 6A, 6B, and an upper portion of theamorphous silicon 4 are etched to define a channel region.

As a third step (FIG. 1C), a passivation layer 7 is deposited onto thestructure of FIG. 1B and a contact hole is formed through a portion ofthe passivation layer 7 using a photolithography process to expose anupper portion of the drain 6B.

As a fourth step (FIG. 1D), an ITO (Indium Tin Oxide) thin film 8 isdeposited onto the structure of FIG. 1C and then patterned to form apixel being connected with the exposed drain 6B.

Hereinafter, the method for fabricating a TFT in accordance with theconventional art will be described in more detail.

First, as depicted in FIG. 1A, a metal is deposited onto the upperportion of the glass substrate 1, a photoresist is coated on the upperportion of the metal, the photoresist coated on the upper portion of themetal is exposed and developed to form a photoresist pattern. The gateelectrode 2 is formed by etching the metal by an etching process usingthe photoresist pattern as an etching mask, and the photoresist patternis removed. Then, the insulating layer 3, the amorphous silicon 4 andthe n⁺ amorphous silicon 5 are sequentially deposited on the abovestructure. A photoresist is coated onto the upper surface of the n⁺amorphous silicon 5, then exposed and developed (using an exposure mask)to form a photoresist pattern at portions opposing the upper andsurrounding areas of the gate electrode 2. Next, an active region isformed at the upper and surrounding areas of the gate electrode 2 bypatterning the n⁺ amorphous silicon 5 and the amorphous silicon 4 usingan etching process employing the photoresist pattern as an etching mask,and the photoresist pattern is then removed using a wet etchant, etchantgas or the like.

As depicted in FIG. 1B, a metal is deposited onto the structure of FIG.1A, and then a photoresist is coated thereon. After forming aphotoresist pattern upon exposure and development, the metal is etchedusing an etching process employing the photoresist pattern as an etchingmask to form a source 6A and a drain 6B which are respectively separatedby a certain distance above the center portion of the n⁺ amorphoussilicon 5 and formed onto a portion of the gate insulating layer 3 atthe sides of the active region.

Then the above etching process is continued so that portions of the n⁺amorphous silicon 5 exposed between the source and drain 6A, 6B, and anupper portion of the amorphous silicon 4 under the n⁺ amorphous silicon5 are etched to define a channel region.

Here, the etching mask that is used allows light to pass through to thechannel region and to the regions adjacent to the source and drain 6A,6B. For example, the mask can have light blocking portions 10 and lighttransmitting portions 20 as shown in FIG. 2, wherein the light blockingportions 10 block light from reaching the source and drain 6A, 6B. Itcan be understood that the light intensity and diffraction amount forthe light passing through each of the light transmitting portions 20formed in the mask is relatively equivalent, as shown in the graph ofFIG. 2.

The minimum line width of the channel region that can be formed by theconventional etching mask depends on the resolution of the exposuredevice. However, using conventional techniques, it is currentlyimpossible or at least very difficult to properly form photoresistpatterns having a line width of less than 4 μm, and thus the channel tobe formed under the photoresist pattern cannot have a length of lessthan 4 μm. The reason for this is because when the light transmittanceregion of the etching mask is less than 4 μm in size, the distributionfor the amount of exposed light passing through the conventional etchingmask is undesirably spread out and the etched portions are thus notsharply defined. As a result, portions of undesired photoresist mayremain (i.e., the photoresist is not properly etched) due to overlappingof adjacent photoresist pattern portions, and thus the desired overallphotoresist pattern may not be properly formed using conventional arttechniques.

Next, as depicted in FIG. 1C, the passivation layer 7 is deposited ontothe upper surface of the structure of FIG. 1B, and an upper portion ofthe drain 6B is exposed by forming a contact hole through thepassivation layer 7 using a photolithography process.

Finally, as depicted in FIG. 1D, a pixel contacting the exposed drain 6Bis formed by depositing an ITO thin film 8 at the upper surface of thestructure of FIG. 1C and the patterning thereof is performed by aphotolithography process.

SUMMARY OF THE INVENTION

The present invention involves the recognition by the present inventorsof the problems in the conventional art. Namely, the conventional artmethod for fabricating a TFT for a TFT-LCD device is problematic in thatthe formation of the channel region width depends only upon theresolution of the exposure device. Conventional exposure devices andconventional TFT fabrication techniques do not allow the channel regionto be formed to be less than 4 μm in length. Thus, when the gate widthof each TFT in high resolution LCD devices is decreased in size so thata greater number of TFTs can be formed, the amount of charge currentbeing supplied for charging the pixels of the TFTs is relativelydecreased as well, due to the smaller gate widths. Due to the smallercurrent being supplied to the pixels, the overall pixel charging time isundesirably delayed. This can be prevented by improving the mobility ofelectrical charges, which is achieved by shortening the length of thechannel when the active region of a TFT is made of amorphous silicon.

Accordingly, it is an object of the present invention to provide amethod for fabricating a TFT, which is capable of forming a channelhaving a length less than 4 μm, which is particularly desirable formanufacturing high resolution TFT-LCDs.

To achieve the above object, the method of the present inventionincludes the steps of forming a gate electrode on an upper portion of aglass substrate and forming an active region over an upper portion ofthe gate electrode and the part of the upper portion of the glasssubstrate; forming a channel region by depositing a metal onto the upperportion of the active region, forming a source and a drain region oneach right and left upper portion of the active region by patterning themetal by a photolithography process, and etching a part of the upperportion of the active region between the source and drain; and forming apixel contacting the drain by depositing a passivation layer on theupper surface of the structure and exposing the part of the drain byforming a contact hole in the passivation layer, wherein thephotolithography process for forming the source, drain and channelregions allows a channel region having a length of 1-4 μm to be formedby using a silt type mask having slits (slots, openings, etc.) thereonthat are positioned to correspond with the channel region to be formed.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIGS. 1A-1D are sectional views illustrating a method for fabricating aTFT in accordance with the conventional art;

FIG. 2 illustrates the conventional mask for forming a source, a drainand a channel region and a graph illustrating an intensity of lighttransmitted through portions of the mask;

FIGS. 3A-3E are sectional views illustrating a method for fabricating aTFT in accordance with the present invention; and

FIG. 4 illustrates a mask in accordance with the present invention and agraph illustrating an intensity of light transmitted through portions ofthe mask.

FIGS. 5A-5E show another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method for fabricating a TFT for a TFT-LCD device in accordance withthe present invention includes forming a gate electrode at the upperportion of a glass substrate and forming an active region at an upperportion of the gate electrode and a part of the upper portion of theglass substrate; forming a channel region by depositing a metal onto theupper portion of the active region, forming a source and a drain regionon each right and left upper portion of the active region by patterningthe metal by a photolithography process, and etching a part of the upperportion of the active region between the source and drain region; andforming a pixel contacting the drain by depositing a passivation layeron the upper surface of the structure and exposing a part of the drainby forming a contact hole on the passivation layer. The source, drainand channel regions are formed by a photolithography process using aslit type mask having a slit thereon positioned to correspond with thechannel region to be formed. The above features will be described inmore detail with reference to the accompanying drawings.

FIGS. 3A-3E are sectional views illustrating fabrication processes of aTFT in accordance with the present invention. The method in accordancewith the present invention includes the following steps. A first step(FIG. 3A) involves forming a gate electrode 2 on a glass substrate 1,then depositing an insulating layer 3, an amorphous silicon layer 4 anda n+amorphous silicon layer 5 injected with high density n-type ionssequentially thereon, and then patterning the n⁺ amorphous silicon 5 andthe amorphous silicon 4 by performing a photolithography process. In asecond step (FIG. 3B) the structure of FIG. 3A is coated with a metal 9and a photoresist PR, then using a mask having a slit portioncorresponding to a central region of the active region to expose anddevelop portions of the photoresist PR. In a third step (FIG. 3C) thephotoresist PR is used as an etching mask to pattern the metal 9 by anetching process to form a source 6A and a drain 6B region which arerespectively separated by a certain distance above the center portion ofthe n⁺ amorphous silicon 5 and formed onto a portion of the gateinsulating layer 3 at the sides of the active region, and then the n⁺amorphous silicon 5 exposed between the source 6A and drain 6B, and anupper portion of the amorphous silicon 4 disposed under the n⁺ amorphoussilicon 5 are etched to define a channel region having a size of lessthan 4 μm. A fourth step (FIG. 3D) is performed by depositing apassivation layer 7 onto the upper surface of the structure of FIG. 3C,and an upper portion of the drain 6B is exposed by forming a contacthole through the passivation layer 7 using a photolithography process. Afifth step (FIG. 3E) is performed by forming a pixel contacting theexposed drain 6B by depositing an ITO thin film 8 on the upper surfaceof the structure of FIG. 3D and patterning it with a photolithographyprocess.

Hereinafter, the method for fabricating a TFT in accordance with a firstembodiment of the present invention will be described in more detail.

First, as depicted in FIG. 3A, the gate electrode 2 is formed bydepositing a metal onto the upper portion of the glass substrate 1 andpattering the metal by a photolithography process.

Next, the insulating layer 3, the amorphous silicon layer 4, and the n⁺amorphous silicon layer 5 injected with high density n-type ions aresequentially deposited onto the upper surface of the glass substrate 1having the gate electrode 2 formed thereon. The n⁺ amorphous silicon 5and the amorphous silicon 4 are patterned by a photolithography processto define an active region at portions above and around the gateelectrode 2 and over portions of the insulating layer 3.

Then, as depicted in FIG. 3B, a metal 9 is deposited onto the upperportion of the structure of FIG. 3A, and the photoresist PR is coatedonto the upper portion of the metal 9.

FIG. 4 illustrates a detailed structure of the mask and a graph showingthe intensity of light being transmitted through the mask according tothe present invention. The mask has light blocking portions 10 and lighttransmitting portions 20, as in the conventional art masks. But, themask of the present invention has a particular configuration for thelight transmitting portion corresponding to where a channel is to beformed. Namely, at least one light blocking portion 11 and a pluralityof light transmitting portions 22 form an opening having at least twoslits (or slots, slit-type openings, etc.), as shown in FIG. 4. Thelight blocking portion 11 blocks portions of light that would otherwisepass freely through the light transmitting portion 20 of theconventional art mask shown in FIG. 2. Accordingly, the intensity of thelight transmitted by the mask of the present invention provides animproved exposure definition (e.g. line width formation accuracy) asevidenced by the steep slopes in the graph of FIG. 4, compared with thelight intensity spread having gradual slopes as shown in FIG. 2. Here,it should be noted that the actual shape of the openings in the maskportion above where the channel is to be formed is not limited to slits.Various other shapes may be employed as long as the light transmittedtherethrough allows an improved exposure definition—to be achieved. Assuch, slots or other slit-type shapes may be employed to achieve thesame results of improved exposure definition.

When light is irradiated onto the photoresist PR by using the mask ofFIG. 4, light with relatively high energy is applied onto the portionsof the photoresist through the light transmitting portions 20 so thatthese photoresist portions are completely removed by the developingsolution applied after exposure. In contrast, the channel regionreceives light with relatively low energy because of the blockagecreated by the light blocking portion 11 (where light can only passthrough the slits, i.e., light transmitting portions 22) so that thephotoresist over the channel region are only partially removed by thedeveloping solution applied after exposure, such that a uniform layer ofphotoresist remains above the channel region.

In other words, the channel formation according to the conventional artcauses over-exposure of the photoresist at a center portion of thechannel with undesired photoresist material remaining at the walls ofthe channel to be formed, while the channel formation according to thepresent invention sharply defines the walls of the channel to be formedwith no over-exposure of the photoresist at a center portion of thechannel to be formed. This allows more accurate removal of thephotoresist in subsequent steps so that the resulting channelconfiguration has a width of less than 4 μm.

After the photoresist PR is irradiated using the mask of FIG. 4 havingslits as explained in the manner above, the exposed photoresist PR isdeveloped with a developing solution.

Next, as depicted in FIG. 3C, the metal 9 is patterned by an etchingprocess using the photoresist PR as an etching mask to thus form aseparate source 6A and a separate drain 6B on the n⁺ amorphous silicon 5and on portions of the insulating layer 3 adjacent to the sides of theactive region.

Here, portions of the metal 9 that are exposed without any photoresistPR pattern thereon (such as the metal 9 portions on the insulating layer3) are etched first. Then, the portions above the channel to be formedare etched because of the remaining uniform photoresist portions createdby light exposure through the slits of the mask of FIG. 3B. As thephotoresist over the source 6A and drain 6B are thicker than the uniformphotoresist over the channel region, etching thereof occurs after thephotoresist above the channel is etched. The etching process isconducted in a manner such that the central portions of 5 is completelyetched, while only the upper, central portion of 4 is etched to definethe channel. As the channel is being defined, the photoresist over thesource 6A and drain 6B are completely etched away. Thus, the structureof FIG. 3C is obtained.

Thereafter, as depicted in FIG. 3D, a passivation layer 7 is depositedonto the structure of FIG. 3C, and a portion of the drain 6B is exposedby forming a contact hole through the passivation layer 7 using aphotolithography process.

Finally, as depicted in FIG. 3E, a pixel contacting with the partiallyexposed drain 6B is formed by depositing and patterning an ITO thin film8 onto the structure of FIG. 3D.

Regarding the formation of the channel, when considering theconventional margin of error range for an exposure process used infabricating TFT-LCDs, it is difficult to form a channel having a lengthof less than 1 μm. In practice, forming a channel having a length in therange of 1 μm˜4 μm is more feasible and realistic according to thepresent invention. More preferably, the channel length should be withinthe range of 2 μm˜3 μm to accommodate a margin of error duringfabrication and to maximize the semiconductor characteristics of thechannel region of a TFT. However, as semiconductor fabricationtechniques continue to evolve and further improve in the future, it canbe foreseen that the present invention technique of employing a maskwith slits (slots, slit-type openings, etc.) that provide a controlledtransmittance of light or energy irradiation onto the channel region mayalso be implemented with more advanced semiconductor fabricationtechnologies under current or future development.

The TFT manufacturing steps of FIGS. 3A-3E described above are sometimesreferred to a so-called “5-mask” process. Namely, five masks are used tofabricate the various portions of the TFT. For example, a mask is usedfor forming the gate electrode, forming the active region, forming thechannel, source and drain regions, forming the hole in the passivationlayer, and forming the pixel electrode. Typically, a “4-mask” processuses only four masks. The masks that are used are similar to those usedin the 5-mask process, but only a single mask is employed for formingthe active region, the channel, the source, and the drain. The method ofthe present invention can also be applied to a so-called “4-mask”process described below with reference to FIGS. 5A-5E.

FIGS. 5A-5E show another embodiment of the present invention. In FIG.5A, a metal is deposited onto a glass substrate 1, which is thenpatterned by a photolithography process to form a gate electrode 2.Then, over the glass substrate 1 having the gate electrode 2 thereon, aninsulating layer 3, an amorphous silicon 4, and a n⁺ amorphous silicon 5injected with high density n-type ions are sequentially deposited.

Thereafter, as shown in FIG. 5B, a photoresist PR is coated over thestructure of FIG. 5A, and light is irradiated through a mask such as theone shown in FIG. 4 to form a photoresist pattern. Then, for thephotoresist pattern portions above the gate electrode 2, diffractionlight rays are applied thereto using an appropriate irradiation devicefor reducing the photoresist thickness compared to the other portions ofthe photoresist pattern covering the remaining portions other than abovethe gate electrode 2. Here, the photoresist portions with a reducedthickness will be referred to as the “thinned photoresist” portions.

Then as shown in FIG. 5C, portions of the stacked layer structurecomprising the metal 9, the n⁺ amorphous silicon 5, and the amorphoussilicon 4 that are not covered by the photoresist PR are etched awayuntil the gate insulating layer 3 thereunder is exposed. At the sametime, portions of the metal 9, the n⁺ amorphous silicon 5, and theamorphous silicon 4 underlying the “thinned photoresist” portions areselectively etched away. Etching of the metal 9 results in the formationof a source 6A and a drain 6B. The complete etching of the n⁺ amorphoussilicon 5, and the partial etching of the amorphous silicon 4 result inthe formation of a channel having a length of less than 4 μm.

As shown in FIG. 5D, a passivation layer 7 is deposited over thestructure of FIG. 5C, and a contact hole is formed through a portion ofthe passivation layer 7 to expose an upper portion of the drain 6B.Finally, as shown in FIG. 5E, an ITO thin film 8 is deposited over thestructure of FIG. 5D, and then patterned to form a pixel that isconnected with the drain 6B.

Accordingly, the techniques of the present invention according to thefirst embodiment employing a “5-mask” process can also be applied to a“4-mask” process described above. By doing so, manufacturing costs canbe further reduced and the fabrication steps can be further simplifiedas compared to those of the “5-mask” process.

As described above, the method for fabricating the TFT for a TFT-LCDdevice in accordance with the present invention allows a channel to beformed with a length of less than 4 μm by employing a slit type mask.Irradiation through the slit type mask onto a channel region effectiveprevents the distribution for the amount of exposed light passingthrough the etching mask from undesirably spreading out, which thusallows the etched channel portions to be sharply defined. Unlike theconventional art, which solely relies upon the resolution of an exposuredevice, the present invention modifies the mask structure to have slits,slots, or other slit-type openings to prevent undesirable spreading oflight passing through the portion of the mask above the channel regionto be formed. Accordingly, as the channel of a TFT can be formed to havea length of less than 4 μm, the amount of charge current supplied to thepixels via the TFT formed by the present invention method can beincreased.

This specification describes various illustrative embodiments of amethod and structure of the present invention. The scope of the claimsis intended to cover various modifications and equivalent arrangementsof the illustrative embodiments disclosed in the specification.Therefore, the following claims should be accorded the reasonablybroadest interpretation to cover modifications, equivalent structures,and features that are consistent with the spirit and scope of theinvention disclosed herein.

1. A thin film transistor (TFT) for a liquid crystal display (LCD)device comprising: a substrate with a gate electrode thereon; an activeregion formed above the gate electrode; a source and a drain formedabove the gate electrode and at the end portions of the active region; achannel formed in the active region, said channel having a length of notmore than 4 μm; and a pixel electrode electrically connected with thedrain so that charge current from the gate electrode can be supplied tothe pixel via the channel.
 2. The thin film transistor (TFT) of claim 1,wherein the channel region has a length of 2 μm to 3 μm.
 3. A thin filmtransistor (TFT) for a liquid crystal display (LCD) device whichcomprises: a substrate with a gate electrode thereon; an active regionformed above the gate electrode; a source and a drain formed above thegate electrode and at the end portions of the active region; a channelformed in the active region, said channel having a shortened and definedlength produced by a controlled transmission of light or energyirradiated onto the channel region; and a pixel electrode electricallyconnected with the drain so that charge current from the gate electrodecan be supplied to the pixel via the channel.
 4. A liquid crystaldisplay (LCD) device containing a thin film transistor (TFT) whichcomprises: a substrate with a gate electrode thereon; an active regionformed above the gate electrode; a source and a drain formed above thegate electrode and at the end portions of the active region; a channelformed in the active region, said channel having a length of not morethan 4 μm; and a pixel electrode electrically connected with the drainso that charge current from the gate electrode can be supplied to thepixel via the channel.
 5. The liquid crystal display (LCD) device ofclaim 4, wherein the channel region of the thin film transistor (TFT)has a length of 2 μm to 3 μm.